1. Field of the Invention
The present invention relates to a vehicular steering apparatus equipped with a disturbance occurrence detection unit for detecting the occurrence of a disturbance to a vehicle.
2. Description of the Related Art
In general, when a vehicle receives a strong crosswind during traveling or come into a rough road surface, the driver's control on the steering wheel of the vehicle would be lost by a disturbance to the vehicle, and the driver's feeling of steering stability might probably be lost.
Accordingly, it is necessary to detect the occurrence of the disturbance in an accurate manner so as to assist the steering torque by means of an electric motor for example to perform stabilization control of the vehicle.
In order to detect the occurrence of a disturbance, a known electric power steering apparatus is provided with a steering torque detection unit that detects the steering torque of a steering system and outputs a corresponding steering torque signal, a steering angle detection unit that detects the steering angle of the steering system and outputs a corresponding steering angle signal, a steering rotational speed detection unit that detects the steering rotational speed of the steering system and outputs a corresponding steering rotational speed signal, and a disturbance steering determination unit that determines a disturbance steering state (i.e., the steering system is under the influence of a disturbance) when the direction of the steering angle signal coincides with the direction of the steering rotational speed signal, and when the direction of the steering torque signal does not coincide with the direction of the steering angle signal or the direction of the steering rotational speed signal (see, for example, a first patent document: Japanese patent application laid-open No. H8-268309).
In addition, another known electric power steering apparatus is provided with a calculation unit that calculates a steering angle change vs steering torque change ratio which is the ratio of an amount of change of a steering torque obtained by a steering torque change amount calculation unit to an amount of change of a steering angle obtained by a steering angle change amount calculation unit, and a disturbance occurrence determination unit which determines that a disturbance to the vehicle has occurred, at the time when the steering angle change vs steering torque change ratio thus obtained is equal to or more than a predetermined value (see, for instance, a second patent document: Japanese patent application laid-open No. 2002-264832).
Also, a further known vehicular steering apparatus is provided with a behavior state detection unit that includes at least one of a yaw rate sensor and a lateral acceleration sensor, and detects the behavior of a vehicle in accordance with the value of a yaw rate or a lateral acceleration detected by the yaw rate sensor or the lateral acceleration sensor (see, for example, a third patent document: Japanese patent application laid-open No. 2000-25630).
In the known electric power steering apparatuses as described in the above-mentioned first and second patent documents, the occurrence of a disturbance to the vehicle is detected based on the sign of the steering torque and the amount of change of the steering torque. However, the steering torque contains friction torque generated in the steering system, so there has been a problem that the occurrence of a disturbance is not able to be detected in an accurate manner in a region in which the steering torque is limited.
In addition, in the known the vehicular steering apparatuses as disclosed in the above-mentioned third patent document, in order to detect the occurrence of a disturbance (the behavior of a vehicle), the yaw rate sensor for detecting the yaw rate or the lateral acceleration sensor for detecting a lateral acceleration is required. However, among vehicles of which the behaviors are disturbed by the occurrence of a disturbance, there are a lot of light cars and compact cars with short wheel bases, so there has been a problem that it is difficult to add either one of the sensors from the point of view of the cost. Further, there has also been a problem that the construction of the apparatus becomes complicated due to the addition of such a sensor.